Telephone operating companies offer many different communication services to business and private customers. Voice services may be offered through the public switched network as well as through private networks. Generally, the private networks are used by large corporations or government agencies to provide both voice and data communications separate from the public switched network.
Analog voice and data service is offered through the private networks to an individual customer by a communications circuit connected between the customer's telephone set and the central office equipment of the telephone operating company. Analog voice and data signals are transmitted between the customer and the central office over the circuit. In order to provide voice communication over the analog service, signalling is required to indicate that service is desired (off-hook) or that service is available (dial tone) and to send dial pulses or tones to indicate busy or ringing as the call is processed.
A persistent problem with using analog service is that analog signals are subject to various types of electrical interference and disturbances which can degrade the service. In large part, however, many of the problems inherent in analog service may be overcome by the use of digital techniques. One such technique is the conversion of the analog signals at the customer site to a digital format suitable for transmission to the central office equipment of the telephone company. The analog signals must be converted to the requisite digital format by a network device installed at the customer location. Once the analog signals are converted to digital format by the network device, the digital data is then sent from the customer to the telephone network in digital form. Since the transmitted digital data is essentially immune to most of the degradation of analog signals, an improved service is realized.
Since conventional voice frequency (VF) subscriber lines are subject to the various problems associated with analog transmission, improvements in service are possible if digital subscriber lines are used to deliver voice frequency services. If analog-to-digital conversion of voice is performed at the customer site by a network device such as an Analog Data Station Terminal, conventional digital subscriber lines can be utilized. A resulting problem, however, is that the transmission of signalling states such as off-hook, dial-tone, and busy-signal must then be accommodated in the digital subscriber line local loop format. This creates a problem in effecting voice communication over the digital subscriber line because the conventional methods of signalling are no longer available. In accordance with the present invention, a method and system are provided to effect reliable signalling transmission over a digital subscriber line.